1. Field of the Invention
The present invention relates to a gray scale signal generating circuit for a thermal printer useful when printing a gray scale image on a colorable thermosensitive paper by supplying a printing signal to a thermal head equipped with a plurality of heating elements. More particularly, this invention relates to a circuit for providing a printing signal to a thermal head even for white level signal intensity. Still more particularly, this invention relates to a circuit which provides a printing drive pulse even if the gray scale intensity is zero.
2. Description of the Prior Art
FIG. 4 is a schematic block diagram of a conventional gray scale signal generating circuit for supplying image data to a thermal printer to print a gray scale image on thermosensitive paper. Input analog image signals are supplied to an input terminal 1 and are digitized by an analog-to-digital (A/D) converter 2 to produce digital image data. The digital image data thus obtained is stored in a field memory 3. The circuit further comprises a line memory 4 where the image data of one line outputted from the digital image data in the field memory 3 is stored and read out. A magnitude comparator 5 sequentially compares the output image data P.sub.D of the line memory 4 with reference gray scale data Q.sub.D and outputs the result of each comparison in the form of a "1" or a "0". A memory (ROM) 6 stores reference gray scale data which is read out by the use of clock signal from a controller 7. A latch circuit 8 receives data from the line memory 4 when the output image data P.sub.D is greater than the reference data. A drive circuit 9 receives the output from the latch circuit 8 to provide a signal to a thermal head 10 pressed against a thermosensitive paper 13 and serving to heat the paper in accordance with printing data supplied to the thermal head 10. The thermal head 10 normally includes a plurality of heating elements numerically equal to the image data inputted to the line memory 4. The controller 7 is fed with a timing signal T from an external source.
In such a thermal printer, when image data to be printed is stored first in the field memory 3 via the A/D converter 2, the image data of one line is supplied to the line memory 4 by a read control signal S.sub.R generated from the controller 7 in accordance with the timing signal T, and the image data P.sub.D in the line memory 4 are compared sequentially with the reference gray scale data Q.sub.D outputted from the gray scale memory 6.
With respect to the reference gray scale data, as shown in FIG. 5 for example, data Q.sub.DO (Q.sub.D = 0) are outputted at an instant to and then image data stored in a line memory 4 is sequentially checked or compared. Thereafter, the signal level is raised stepwise with the lapse of time t as shown by the curve in FIG. 5. Such reference data at the individual levels is compared with the incoming image data P.sub.D of one line outputted from the line memory 4. The results of the individual comparisons are held sequentially in the latch circuit 8.
Data is thus obtained at a high (H) level during the period where Q.sub.DO (O) &lt;P.sub.D .ltoreq.Q.sub.DY. Therefore, when the image data has a level P.sub.D1 as shown in FIG. 5, Q.sub.D =P.sub.D1 at an instant tl, so that printing data SD1 is turned to a low (L) level at the instant t.sub.1 is outputted from the latch circuit 8.
Similarly, when image data P.sub.D2 is obtained, Q.sub.D =P.sub.D2 at an instant t.sub.2 as shown, so that printing data S.sub.D2 is turned to a 1ow level at the instant t.sub.2 is outputted from the latch circuit 8.
In this manner, the n printing data S.sub.D1 - SDn processed through pulse width modulation (PWM) in conformity with the magnitude of the n image data P.sub.D1 - P.sub.Dn introduced into the line memory 4 is obtained on the basis of the comparison results from the magnitude comparator 5. The printing data is supplied via the latch circuit 8 to the drive circuit 9 which consists of a switching circuit.
The drive circuit 9 causes a flow of printing currents of different pulse widths in the individual heating elements of the thermal head in accordance with the printing data S.sub.D1 - S.sub.Dn, so that the heating time is rendered longer for the image data of a higher density. The thermosensitive paper temperature is raised in a monochrome printer to consequently form a blacker color. For the image data of a lower density, the heating time is rendered shorter to induce gray printing.
Therefore, a gray scale image can be printed when such printing operation is performed over the entire image area of one field while feeding the thermosensitive paper. The modulation system based on the above-described technique is disclosed in U.S. Pat. No. 4,399,749 filed previously by the present applicant. That patent is herein incorporated by reference for the sake of disclosure.
In the thermal printer mentioned, as shown in FIG. 6 (a), the lightest portion is set to the white level D.sub.min of the printing paper while the darkest portion is set to the maximum coloring level D.sub.max of the paper so that the density dynamic range D.sub.max - D.sub.min can be widened relative to the gray scale signal N.
The relationship between the density D of the thermosensitive paper and the heating energy E, as shown in FIG. 6(b) generally has such an inclination that the rise is extremely unsatisfactory in the low density region, as shown especially in the circle in the lower left portion of the figure. In contrast, a saturation is induced in the high density region near D.sub.max.
Since the density-to-heating energy curve is changed with the ambient temperature fluctuation and the heat storage state of the thermal head as represented by a one-dot chain line, the printing data supplied to the thermal head need to be controlled so that the density of the print image is not varied by the ambient temperature fluctuation. However, as shown in an enlarged graphic representation of FIG. 6 (c), the sensitivity characteristic is such that, in the vicinity of the region where the density becomes zero, this indicating a white level, the curve B is changed remarkably by the harmful influence of the temperature fluctuation. There occurs a steep rise as indicated by a curve A or a sharp rise immediately after the white non-sensible region as indicated by a curve C.
When the density curve varies in the intermediate step of the gray scale, substantially no image deterioration occurs that brings about a problem in the printing area. However, once a great density difference is caused in the gray scale 1 or 0 where the density is zero, a false contour appears in the printing area.
In the case of the curve C, for example, the densities at the gray scales 0, 1 and 2 become the same. Thus, there exists a problem that the quality of the printed image is extremely deteriorated at these low densities.